1. Field of the Invention
The present invention concerns substrates for soil-free culture, in particular substrates which have a water content controlled over their thickness.
2. Background of the Prior Art
It is important, for the growth of plants, that the substrate medium contains air and can absorb and retain water or nutrient aqueous solutions.
There have been proposed, for soil-free culture, substrates based on mineral fibres, such as rock wool or glass fibres, because these substrates have the advantage of being highly porous, the fibres occupying generally at most 5% of the total volume of the substrates. They can act as supports for roots, as a reserve for air and water or nutrient solutions; furthermore they are light and chemically inert.
The present description will refer, for convenience, to the supply of water to the substrates; however, it is to be understood that all aqueous nutrient solutions suitable for plants may be used.
The retention of water, or hydroretention, of a substrate is an important characteristic for the procedure of plant culturing. The conditions of humidity can vary according to various factors such as the type of plants to be cultivated, the climate, the seasons, and the stage of development of the plant. Whatever the desired humidity conditions for a particular type of substrate, it is necessary for the water to be absorbed and retained to a certain extent by the substrate. The water should not drain away immediately, but rather it should remain available to the plant. In fact, water or an aqueous solution which is too strongly bound to the substrate or which drains away too quickly cannot be satisfactorily utilised by the plant.
The substrates based on mineral fibres, as a result of their characteristics, give satisfactory results when they are used for soil-free culture. However, they still display one disadvantage; at the time of use the substrates are positioned on impermeable supports and are generally supplied with water by percolation. The water flows under gravity across the substrate. The lower part of the substrate remains in contact with the water by virtue of its capillary nature. The lower part of the substrate thus contains more water and less air than the upper part. The characteristics of water/air distribution are thus not the same over the whole thickness of the substrate; the roots develop in a non-uniform manner within the substrate as a whole; which is harmful to the plant and reduces the effectiveness of the substrate.